Sulfur-containing prepolymer such as polythioether prepolymers and polysulfide prepolymers are known to be useful in aerospace sealant applications. Aerospace sealants must meet a number of challenging performance requirements that including adhesion, tensile strength, elongation, fuel resistance, and high temperature stability. Typical sulfur-containing prepolymer-based sealants are characterized by a relatively short working time of less than 12 hours and can take several weeks to fully cure. For example, aerospace sealants based on thiol-epoxy reactions can be catalyzed by amines. Strong base catalysts such as 1,4-diazabicyclo[2.2.2]octane exhibit a working time of only a few hours. Weak amine catalysts are ineffective in providing a cured product.
Sulfur-containing-based sealant formulations exhibiting extended working time and that cure rapidly to a hardness of at least 30 Shore A at the end of the useful working time are desired.
These objectives can be achieved using latent amine catalysts such as disclosed, for example, in U.S. Pat. No. 9,006,360, which is incorporated by reference in its entirety. However, encapsulated catalysts can be difficult to manufacture and the quality can be difficult to maintain on a commercial scale. Blending encapsulated catalysts homogenously throughout a sealant can also present challenges. For example, heat and/or shear generated during mixing can prematurely rupture and/or release the amine catalysts. Once mixed, the catalyst can also diffuse from the encapsulant causing premature curing. Finally, the encapsulating material can compromise the properties of the cured sealant.
Thiol-epoxy systems having a working time of one to three (1 to 3) weeks, that rapidly cure to a hardness of at least 30 Shore A after the end of the working time, and that meet the demanding performance requirements of aerospace sealants when fully cured are desired.